Shunt Resistors Used with Ammeters: Quiz and Flashcards

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Questions and Answers

What is the main advantage of using a shunt resistor in a moving coil ammeter?

It decreases the thermal electromotive force

It improves the accuracy of the instrument

It increases the current capacity of the instrument (correct)

It reduces the internal resistance of the movement

Why is a permanent-magnet moving coil (PMMC) instrument impractical for measuring large currents directly?

The resistance of the shunt varies with time

The thermal electromotive force with copper is too high

The current capacity of the PMMC is too small (correct)

The coil cannot carry large currents without excessive temperature rise

What is the relationship between the shunt resistance ($R_{sh}$) and the internal resistance of the movement ($R_m$) in a moving coil ammeter?

$R_{sh} = R_m / (I / I_m - 1)$ (correct)

$R_{sh} = R_m / (I_m - 1)$

$R_{sh} = R_m / (I - I_m)$

$R_{sh} = R_m$

Which of the following is NOT a desirable property of a shunt resistor for a moving coil ammeter?

It should have a high thermal electromotive force with copper Signup and view all the answers

What is the purpose of the 'multiplying power of the shunt' ($I/I_m$) in a moving coil ammeter?

It converts the current reading to the actual current being measured Signup and view all the answers

How can the current range of a moving coil ammeter be extended to measure higher currents?

By connecting a shunt resistor in parallel with the instrument movement Signup and view all the answers

What is the main advantage of a moving coil instrument compared to other types of instruments?

Greater sensitivity and precision Signup and view all the answers

Which of the following equations correctly represents the relationship between the full-scale voltage ($V$), the deflection current ($I_m$), the internal resistance ($R_m$), and the series resistance ($R_s$) for a voltmeter?

$V = I_m (R_s + R_m)$ Signup and view all the answers

What is the purpose of using a shunt resistor in an ammeter?

To increase the range of the instrument Signup and view all the answers

Which of the following equations correctly represents the relationship between the shunt resistance ($R_{sh}$), the internal resistance ($R_m$), and the full-scale deflection current ($I_m$) for an ammeter?

$R_{sh} = R_m / (I_m / (I_m - 1))$ Signup and view all the answers

What is the main disadvantage of using a moving coil instrument for high-voltage measurements?

Requirement of a multiplier resistor Signup and view all the answers

Which of the following equations correctly represents the relationship between the full-scale voltages ($V_1$, $V_2$, $V_3$, $V_4$) and the internal resistance ($R_m$) for a multi-range voltmeter?

$V_2 = V_1 - I_m R_m$, $V_3 = V_2 - I_m R_m$, $V_4 = V_3 - I_m R_m$ Signup and view all the answers

What is the main purpose of the two springs being spiraled in opposite directions in a moving coil instrument?

To neutralize the effects of temperature changes Signup and view all the answers

Which of the following is NOT an advantage of a permanent-magnet type moving coil instrument?

Ability to be modified to cover a wide range of currents and voltages Signup and view all the answers

Which of the following is a disadvantage of permanent-magnet type moving coil instruments compared to moving-iron instruments?

More delicate construction and costlier Signup and view all the answers

What is the main purpose of using a shunt with an ammeter?

To extend the range of the ammeter Signup and view all the answers

Which of the following is NOT a desirable property of a good shunt for an ammeter?

Uniform scale Signup and view all the answers

Study Notes

Ideal Shunt Characteristics

The resistance of a shunt should not vary with time.
Shunts should carry current without excessive temperature rise.
They should have a thermal electromotive force (EMF) with copper.

Ammeter Shunt

A PMMC (Permanent Magnet Moving Coil) is used as the indicating device in an ammeter.
The current capacity of a PMMC is small, making it impractical to construct a PMMC coil that can carry a current greater than 100 mA.
A shunt is required for measuring large currents in an ammeter.

Shunt Calculation

The voltage drop across the shunt (Rsh) and movement (Rm) must be the same.
The equation to calculate the shunt resistance (Rsh) is: Rsh = Im * Rm / (I - Im)

Multi-Range Ammeter

A multi-range ammeter uses multiple shunts with different multiplying powers (m) to measure different ranges of currents.
The shunt resistance for each range is calculated using the equation: Rsh = Rm / (m - 1)

Voltmeter

A series resistor or multiplier is required to extend the range of a voltmeter.
The equation to calculate the multiplier resistance (Rs) is: Rs = (V - Im * Rm) / Im

Multi-Range Voltmeter

A multi-range voltmeter uses multiple multipliers with different multiplying powers (m) to measure different ranges of voltages.
The multiplier resistance for each range is calculated using the equation: Rs = Rm * (V - Im * Rm) / Im

Examples

Example 1: Calculating the shunt resistance for an ammeter with a full-scale deflection current of 1 mA and an internal resistance of 100 ohms.
Example 2: Calculating the series resistance for a voltmeter with a full-scale deflection current of 1 mA and an internal resistance of 100 ohms.
Example 3: Calculating the full-scale reading of a PMMC instrument.

PMMC Instrument Advantages

They have low power consumption.
Their scales are uniform and designed to extend over an arc of 170°.
They possess a high (torque/weight) ratio.
They can be modified with the help of shunts and resistances to cover a wide range of currents and voltages.
They have no hysteresis loss.
They have very effective and efficient eddy-current damping.
They are not much affected by stray magnetic fields.

PMMC Instrument Disadvantages

They are somewhat costlier due to delicate construction and accurate machining and assembly of various parts.
Errors set in due to the aging of control springs and the permanent magnets.
They are mainly used for DC work only, but can be used for AC measurements with rectifiers or thermo-junctions.

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Description

Learn about the important characteristics and functions of shunt resistors used in ammeters. Understand why shunt resistors should not vary with time, carry current without excessive temperature rise, and have a thermal electromotive force with copper. Explore the necessity of using shunt resistors to measure large currents in ammeter circuits.

Shunt Resistor in Ammeters